Protection of molybdenum against oxidation



egpf W53 J. w. GARRISON ET A1. 2,650,903

PROTECTION OF' MOLYBDENUM AGAINST OXIDATION Pw En 'Filed July 5, 1947 Patented Sept. 1A,

UNITED PATENT OFFICE y Y Y Y 2,650,903 PROTECTION FV MOLYBDENUM AGAINST OXIDATION hihi; riparata Arnngfbn, ima Albert e. Loyett, East Orange, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Par, erpatioii of Pennsylvania ispit'ion july 5, 1947, serial No. 759,256 '9 claims. (c1. en -3s) This invention relates to the manufacture of articles of molybdenum and alloys consisting mainly of molybdenum protected against oiidation by a surface coating. I

The principal object of -our invention, generally considered, is the application to articles formed of molybdenum and alloyein which molybdenum is the main constituent, either with or without a preliminary coating of cobalt, of a uniform silica-type coating, that is, one consisting large- 1y of silica, red thereon at a temperature above 130G C.

Another object of our invention is to apply a uniform silica-type. coating to molybdenum or an alloy thereof, either with or without preliminary plating of cobaltthere'on, and then fire to cause the coating to fuse on the surface as a uniform non-porous nlm.

A further object of vour', inyention is to protect articles of molybdenum or alloys thereof by mak'- ing a slip of silica and glass in an alcoholic medium, screening, and spraying orvdippinguto apply said slip to the articles which are then dried and fired to cause fusion of the coating material, whereby non-porous uniform protective coatings are caused to rmly adhere to the articles.

A still further object of our invention is to protect molybdenum and alloys thereof by a silica-type coating superimposed on a coating of molybdenum silicide, thereby at the same time increasing the hardness approximately 20 points on the Rockwell B scale and greatly decreasing oxidation, while having a coating whichyolatizes little, if any, at temperatures up to about 10G()c C`.

Other objects and advantages of the invention will become apparent as the description proceeds. v

Referring to the drawing: Y

Figure l is a diagrammatic sectional View, partly in elevationI and partly in vertical section, showing apparatus used in the process of making the slip for protecting molybdenum articles.

Figure 2 is a diagrammatic sectional view showing apparatus ueed in the process of screening the slip to remove coarse particles` prior to application to the article to be protected.

Figure 3 is a diagrammatichyiew of apparatus including a container for the slip, means for stirring a slip therein, and a spray gun for applying said slip to the article t'o be protected.

Figure 4 is a view similar to Figure y3, but showmg apparatus which may be used for appiying the slip to the article to beprotected' by means of dipping. i l

Figure 5 is' atrayese' 4sectionalvieii Iof 2 oven which may be used for drying the slip on the article to be protected prior to fusing it in place.

Figure 6 is an elevational View, partly in vertical section, showing a furnace which may be used for fusing the dried slip on the metal article to be protected.

Figure 7 is a transverse sectional view of an article formed of molybdenum or alloy thereof, either with or without a plating of cobalt, and protected by a silica-type coating.

For several years, various attempts have been made to apply protective coatings to molybdenum and alloys consisting mainly of molybdenum, for the purpose of resisting high temperature oxidation. Such treatment as chromizing, chromium plating, nitriding, calorizing, nickel plating, platinum plating, and others have been tried without success. Attempts to form surface coatings such as berate, borosilicate, and various other types were made. Some evidence of resistance to heavy volatilization of molybdenum 0X- ide above 750 C. was shown by a silica-type coating formed by packing a piece of molybdenum in a mixture of silicon and silica and treating at 1000 C. in dry hydrogen, as determined by us.

Since at approximately 1300 C., silicon and hydrogen form hydrogen silicide which is reactive, a protective coating of molybdenum silicide, a stable compound, is formed by such a treatment of molybdenum with a silica-type or high silica glass coating thereon.

This invention, therefore, relates to the application of such a coating and the subsequent ring thereof to provide a non-porous uniform coating protective against oxidation for-molybdenum, and similar material such as alloys consisting mainly of that metal.

Referring to the drawing in detail, like parts being designated by like reference characters, there is shown a container l l holding a quantity of alcohol l2, which may be of the methyl type, and into which is beingadded aquantity of silica I3 and a quantity of glass powder I4. The purpose of mixing silica with glass is to increase the silica content of the glass mixture, in order to provide material which is very Viscous when hot. A metal article coated with such material will not have the coating disturbed or blown therefrom, even when heated to a temperature high enough to melt said coating. A desirable formula for the mixture being made, as shown 5s. in Figure l, is in the following proportions, parts 0.5; CaO, 6.5; MgO, 1.9; NazO, 3.0; and KzO, 1.6.

102.3 parts by weight or pounds of glass of the foregoing composition is desirably made by agitating the following ingredients in a pony mixer, or other device for thoroughly incorporating them, then placing in small sand crucibles and holding in a pot furnace at about 1350 C. until melted. A desirable raw batch formula is as follows:

Parts by weight or pounds Silica, S102 58.6 Aluminum hydroxide, Al(OH)a 39.5 Barium carbonate, BaCO3 12.5 Calcium carbonate, CaCO3 11.7 Magnesium carbonate, MgCO3 4.5 Zinc oxide, ZnO 0.5 Sodium carbonate, Na2CO3 5.2 Potassium carbonate, K2CO3 2.5

After cooling, the glass is ground and silica added. A preferred ratio between the glass, powdered silica, and alcohol is 100 grams glass powder, 20 grams powdered silica, and 150 cc. of alcohol. A glass melt is made and, after cooling and grinding, the desired additional silica added to make the desired final composition, in order to facilitate melting the glass. If all the desired silica were in the original mix, a very high ternperature would be required for melting it. The final mixture, containing between 16 and 17% or about 16.7% of alumina, is agitated, as by means of a stirring rod I5, and milled for preferably about 24 hours in order to insure that the materials are very finely divided and thoroughly mixed.

After the formation of the slip, the same is desirably passed through a 325 mesh screen I6, as indicated in Figure 2, in order to remove any coarse particles, the screened slip being collected in receptacle I'I. The slip is then applied to an article I8 of molybdenum or molybdenum alloy, as by means of a spray gun I9 fed through tube 2I from the slip I2, while the same is desirably stirred by means of a power-driven impeller 22. As an alternative, the slip I2 may be applied to the article I8 by dipping the latter therein, While said slip is being stirred as by power-driven impeller 22, as in Fig. 4.

Instead of applying the slip directly to the molybdenum surface of the article I8, the latter is desirably coated rst with a film of cobalt, as by means of electroplating. A desirable formula for the electroplating bath is:

. Grams CoSOi-7H2O 504 NaCl 17 H3BO3 45 Water to make 1 liter The operating conditions for electrodepositing cobalt from such a bath on molybdenum or molybdenum alloy articles, are as follows:

Temperature 20-30 C. Current density 30-164 amps/ft.2 Anode 99% cobalt. Time 30 sec.

After application of the silica-type coating to the plain or cobalt-plated molybdenum article, as above described, the article is either permitted to dry naturally or placed in a drying oven 23, as shown in Figure 5, for that purpose.

After drying, the coated article is passed through a hydrogen retort furnace 24, as illustrated in Figure 6, said article being slightly preheated in the front entrance chamber 25, and then passed on to the insulated retort 26, where it is heated as by means of a resistance coil 2l of molybdenum or tungsten around an inner refractory tube 28, while hydrogen is passed thereinto as through pipe 29 and escapes as indicated at 3|. After heating to fuse the coating at a temperature above 1300 C. to effect a reduction of some of the silica in the coating to form hydrogen silicide which, in turn, reacts with the surface of the molybdenum to form a coating of molybdenum silicide under the glass coating, the article is passed through the cooling chamber 32 and finally Withdrawn at the exit 33.

Figure 7 is a cross-sectional view showing a base metal, such as molybdenum or cobalt-plated molybdenum having a protective silica-type coating 34, which may include molybdenum silicide if the fusion temperature in hydrogen is above 1300 C.

A study of coatings applied to articles in the manner above described shows that an apparently non-crystalline outer coating about .004 thick is formed on the surface of the article. Such a photomicrograph, at a magnification of 75, indicates that while there is a definite line of demarcation shown, between the coating and the molybdenum, beyond this line there is a penetration zone approximately .008 in depth having a structure similar to that of an intermetallic compound.

The coating increases the hardness of the article 20 points on the Rockwell B scale over the normal hardness of the molybdenum itself. This increase in surface hardness is expected to improve the resistance to abrasion of the coated material as compared with the uncoated metal.

Tests on coated samples show little if any eVidence of volatilization of the coating at temperatures up to about 1000 C. It is therefore believed that the invention will have considerable use in connection with operations Where it is desired to employ articles of molybdenum or alloys thereof having approximately the characteristics of molybdenum.

Although preferred embodiments of our invention have been disclosed, it will be understood that modifications may be made within the spirit and scope of the appended claims.

We claim:

1. The method of protecting articles of molybdenum and alloys thereof consisting mainly of molybdenum against oxidation, comprising forming a slip in alcohol of nely divided glass and silica, applying a coating of said slip to the surface of the metal to be protected, drying, and fusing said slip on said surface in a hydrogen atmosphere at a temperature above 1300c C. and high enough to generate hydrogen silicide and cause it to react with the surface of said article.

2. The method of protecting articles of molybdenum and alloys thereof consisting mainly of molybdenum against oxidation, comprising placing said article in a solution of a salt of cobalt, electroplating cobalt from said solution on said article, making a slip of finely-divided silica and glass, applying a coating of said slip to the plated article, drying, and fusing said coating thereon in a hydrogen atmosphere at a temperature above 1300? C. and high enough to generate hydrogen silicide and cause it to react with the surface of said article.

3. The method of protecting articles of molybdenum and alloys thereof consisting mainly of molybdenum against oxidation, comprising making a slip of nely divided viscous glass in an alcoholic medium, applying said slip to such an article, drying, and heating in hydrogen to a temperature above 1300 C. to fuse said slip on said article, and cause the evolution of hydrogen silicide and its reaction With the surface of said article, thereby providing a uniform non-porous coating of molybdenum silicide thereon.

4. The method of protecting articles of molybdenum and alloys thereof consisting mainly of molybdenum against oxidation, comprising placing said article in a solution of a salt of cobalt, electroplating cobalt from said solution on said article, making a slip of nely-divided silica and glass with a relatively-high alumina content, applying a coating of said slip to the plated article, drying, and fusing said coating thereon in a hydrogen-including atmosphere at a temperature above 1300 C.

5. The method of protecting articles of molybdenum and alloys thereof consisting mainly of molybdenum against oxidation, comprising making a slip of finely divided high silica glass, containing between about 16% and 17% of alumina, in an alcoholic medium, applying said slip to such an article, drying, and heating in hydrogen to a temperature above 1300a C. to fuse said slip on said article, and cause the evolution of hydrogen silicide and its reaction with the surface of said article, thereby providing a uniform non-porous coating of molybdenum silicide thereon.

6. The method of protecting against oxidation articles of molybdenum and alloys thereof consisting mainly of molybdenum, comprising forming a slip of finely divided glass and silica, applying a coating of said slip to the surface of the article to be protected, drying, and fusing said slip on said surface in a hydrogen atmosphere at a temperature above 1300 C., in order to generate hydrogen silicide and cause it to react with the surface of said article.

7. The method of protecting against oxidation articles of molybdenum and alloys thereof consisting mainly of molybdenum, comprising electroplating cobalt on such articles, making a slip of finely-divided high-silica glass, applying a coating of said slip to the plated article, drying and fusing said coating thereon in a hydrogen atmosphere at a temperature above 1300 C., to form hydrogen silicide and cause it to react with the surface of said article.

8. The method of protecting against oxidation articles of molybdenum and alloys thereof con- Sisting mainly of molybdenum, comprising making a slip of finely divided glass, applying said slip to said articles, drying and heating in hydrogen to a temperature above 1300 C. to fuse said glass on said articles and effect the evolution of hydrogen silicide and its reaction with the surface of said articles.

9. The method of protecting against oxidation articles of cobalt-plated molybdenum and alloys thereof consisting mainly of molybdenum, comprising applying a coating to such plated articles 0f finely-divided high-silica glass suspended in a liquid, drying, and fusing said coating thereon in a hydrogen-including atmosphere at a temperature above 1300" C.

JOHN W. GARRISON. ALBERT B. LOVETT.

References Cited in the file 01" this patent UNITED STATES PATENTS Number Name Date 533,945 Cochran Feb. 12, 1895 1,422,443 Hoge July 11, 1922 1,869,025 Seastone July 26, 1932 1,951,039 Scharschu Mar. 13, 1934 2,071,533 Ihrig Feb. 23, 1937 2,101,950 McGohan Dec. 14, 1937 2,175,689 Gallup Oct. 10, 1939 2,304,297 AntOn Dec. 8, 1942 2,307,801 Pierce Jan. 12, 1943 2,398,712 Malcolmson et al. Apr. 16, 1946 2,418,932 Harr Apr. 15, 1947 2,492,682 Carpenter Dec. 27, 1949 2,510,071 Chester June 6, 1950 2,555,372 Ramage June 5, 1951 FOREIGN PATENTS Number Country Date 731,233 France May 24, 1932 214.492 Switzerland July 1,6. 1941 

2. THE METHOD OF PROTECTING ARTICLES OF MOLPBDENUM AND ALLOYS THEREOF CONSISTING MAINLY OF MOLYBDENUM AGAINST OXIDATION, COMPRISING PLACING SAID ARTICLE IN A SOLUTION OF A SALT OF COBALT, ELECTROPLATING COBALT FROM SAID SOLUTION ON SAID ARTICLE, MAKING A SLIP OF FINELY-DIVIDED SILICA AND GLASS, APPLYING A COATING OF SAID SLIP TO THE PLATED ARTICLE, DRYING, AND FUSING SAID COATING THEREON IN A HYDROGEN ATMOSPHERE AT A TEMPERATURE ABOVE 1300* C. AND HIGH ENOUGH TO GENERATE HYDROGEN SILICIDE AND CAUSE IT TO REACT WITH THE SURFACE OF SAID ARTICLE. 